1. Field of the Invention
The present invention relates to a water-absorbing resin and a method of preparing the same.
2. Description of the Related Art
A super-absorbent polymer (‘SAP’) is a resin capable of absorbing water of several tens to thousands of times the weight of the resin, and refers to a polymer material having a hydrophilic three dimensional net-mesh structure that does not release absorbed water even when a pressure applied thereto in some degree. In early days, there were many super-absorbent polymers using natural polymer such as starch, but in recent years, a synthetic polymer prepared by polymerizing acrylic acid or acrylamide with a small amount of cross-linking agent is mostly used. Currently, a novel absorbent resin capable of absorbing water of several tens to several hundreds of times the weight of the resin has been developed and broadly used in various applications requiring water absorption or water retention including hygienic products such as diapers, sanitary goods, etc.
In a polymerization process among processes of preparing the super-absorbent polymer, a number of oligomers having insufficiently increased molecular weight (waster-soluble fraction, ‘extractables’) may be generated in the resin. If a large quantity of the extractables exist in the resin, these may be dissolved in water or the like, when the resin is actually applied to products, hence causing a problem of being easily dissolved in water or body fluid and eluted out of the products. Accordingly, there is a need for reduction of extractables.
During polymerization of a base resin for a water-absorbing resin, the cross-linking agent has higher reactivity than that of the acrylic acid. Therefore, the cross-linking agent is completely exhausted at an early stage of the reaction, while a polymerization of acrylic monomers may occur at a later stage of the reaction to generate the extractables. Accordingly, it is required to solve a problem that the cross-linking reaction is completed at the early stage thereof.
In addition, there is still a need for improving absorption rate and dry efficiency of the water-absorbing resin. Among solutions for improvement thereof, a foaming technique during the polymerization is known in the related art. This method generally includes addition of a foaming agent or use of an additive for controlling a foaming time and/or a particle diameter of foams. However, this method also entails problems of an increase in production costs and requiring a complicated process.